Carbon-carbon composites are sufficient for general applications, but one important disadvantage in using carbon-carbon composites is that the composites are susceptible to oxidation and are not easily machined into intricate parts having complex geometries. Even more desirable than carbon-carbon composites are fiber-reinforced ceramic matrix composites (CMCs) possessing high strength/weight ratio along with high stiffness/weight ratio, high temperature strength, resistance to thermal shock, impact and fatigue. In the past, fiber-reinforced CMCs have been fabricated primarily by hot pressing of tape materials or by surface coating the fibrous materials. Reinforced fibers are combined with a matrix material into a tape intermediate. This tape intermediate is laminated on a ply-by-ply basis to form the final composite structure. U.S. Pat. No. 4,837,230 to S. Chen et al. discloses the making of CMCs by impregnating a reinforcing fabric with a liquid preceramic resin, e.g. a polycarbosilane, and then curing and pyrolyzing the impregnated structure.
Another technique used to overcome the above-mentioned deficiencies of fiber-reinforced ceramics has been to apply a sealant to the carbon-graphite composites. These carbon-graphite composites have been impregnated with a sealant of boron oxide and a refractory oxide precursor, such as an elemental metal. Additionally, another technique to overcome these deficiencies is to use chemical vapor deposition to apply coatings of silicon nitride onto a substrate using SiH.sub.4 /NH.sub.3 or SiCl.sub.4 /NH.sub.3 in carriers. Chemical vapor deposition techniques are typically used to fabricate passive layers and dielectric coatings for consistent electrical, mechanical and etch properties.
Chemical vapor infiltration (CVI) has also been used to produce fiber-reinforced CMCs as described in U.S. Pat. No. 4,895,108, issued Jan. 23, 1990, to A. J. Caputo et al. However, prior to the invention herein, there were many difficulties associated with employing chemical vapor infiltration to make CMCs. In the past it was necessary to use an elaborate apparatus, where the flexible fibrous material was laid or supported on a grid with perforated or specifically gated plates to support the assembly therebetween. The incoming gases were then channeled to cover the entire surface of the plate. Without channeling the gases, the resulting structure was not uniform. For example, the '108 patent describes an elaborate, water-cooled, gas-distributing mandrel which is used.
In general, the techniques used in the past have resulted in fiber-reinforced ceramics that lack toughness, which has inhibited their use in broad applications. It is quite desirous to overcome such application difficulties to meet the needs of more sophisticated industries such as the aerospace industry. In the aerospace industry, there is a demand for materials that have high-temperature capability and superior mechanical properties on a density-normalized basis at elevated temperatures, and a demand for materials which can be shaped into intricate articles easily without the use of elaborate apparatus.